Clean up print statements for geogrid optional fields

[mgduda]

The changes introduced in PR #16 enable users to not select optional fields, even if those fields are available on disk. The print statements generated by these changes could be made a little cleaner: for example, the output if LAI12M is made optional and not selected looks like:

  Optional fields not processed by geogrid:
    LAI12M (priority=1, resolution='', path='')
    IMPERV (priority=1, resolution='default', path='/sysdisk1/duda/WRF/geog/nlcd2011_imp_ll_9s/')
    CANFRA (priority=1, resolution='default', path='/sysdisk1/duda/WRF/geog/nlcd2011_can_ll_9s/')

[mgduda]

Just for reference, here's the type of message we get when a higher-priority entry is optional and not used:

Parsed 26 entries in GEOGRID.TBL
Processing domain 1 of 2
  Processing XLAT and XLONG
  Processing MAPFAC
  Processing F and E
  Processing ROTANG
  Processing LANDUSEF
   Important note: could not open input dataset for priority level 2, but this level is optional.
  Calculating landmask from LANDUSEF ( WATER =  17  21 )
  Processing HGT_M
  Processing SOILTEMP